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| Defenses | |||
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| Nutritional content | |||
Defenses of nudibranchs and their relatives include nutritional content, considered in this section, and |
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Research study 1 |
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In support of this idea, lower organic content has a significant negative effect on feeding on artificial diets by predatory crabs Cancer productus in no-choice tests in the laboratory (see graph on Right). The graph indicates a significant depression in feeding by the crabs on artificial diets (powdered squid set in alginate gel) containing 12% dry-mass organic content versus ones containing 22% dry-mass organic content. Simply put, the strategy being demonstrated is one of a prey being “just not worth eating”. The author concludes that reduced organic content per bite (equated in the paper with reduced nutritional quality) may offer anti-predator deterrence to nudibranchs independent of any chemical defenses that they may have. NOTE the graph also shows that with the exception possibly of foot and viscera, whether a nudibranch has spicules or not appears to have little effect on its overall organic content. Moreover, the areas of non-significant differences, “margin” and “center”, are the ones likely to be encountered first by a predator. Spicules as defense for nudibranchs are considered elsewhere in this section of the ODYSSEY: NUDIBRANCH DEFENSES: SPICULES NOTE many nudibranchs contain secondary metabolites that may function in defense, as shown in: NUDIBRANCH DEFENSES: SECONDARY METABOLITES |
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An interesting idea relating to defenses of nudibranchs is that, like crinoids and other calcareous invertebrates, there is a large component of non-nutritional matter in their bodies. For example, a comparison at Bamfield Marine Sciences Centre, British Columbia of soft tissues of 3 species of shelled orthogastropods with 7 species of nudibranchs (4 spicule-containing species and 3 non-spicule-containing species) shows a 2-3-fold lesser organic content in the 
nudibranchs